Rectangle station marker overlaps with its previous version until the next call to `drawCanvas()`

[shannonturner]

If a singleton square station is replaced with a non-singleton square station (which is smaller), there's an overlap until the next drawCanvas() call:

One potential solution would be to check for a station in the neighboring 8 points of the just-drawn point; if so, call drawCanvas() for stations only.

Depending on how I address #46, that may address this too.

To reproduce:

• Make a singleton point, then add a square station and notice that the station is a square
• Connect lines to that point, then notice that the station marker shrinks, but the older square station marker from step 1 is still visible
• Change the direction that the rectangle station marker should be drawn by drawing more lines and notice that the overlap occurs again, in a diagonal direction